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Silver cyanide, AgCN

A white, amorphous precipitate of the Silver cyanide, AgCN, is obtained by interaction of a silver salt and a cyanide in aqueous solution. It crystallizes from a hot concentrated solution of potassium carbonate in fine needles. It is unaffected by light, but heat eliminates one-half of the cyanogen, with production of silver "paracyanide." Both hydrochloric acid and mercuric chloride convert it into silver chloride. With hydrogen sulphide it yields Silver sulphide, and heating with sulphur transforms it into silver thiocyanate. Its heat of formation from silver and cyanogen is 3.6 Cal. There is some evidence of the existence of silver cyanide in two polymeric forms, AgCN and Ag2(CN)2. With hydrazine cyanide it forms colourless crystals, AgCN,N2H4, which blacken in contact with air.

Solution of silver or its cyanide in potassium cyanide forms potassium silver cyanide, KAg(CN)2, octahedral crystals stable in air, but blackened by light. As 20° C. its solubility is 25 grams per 100 grams of water.

Its heat of formation from the two cyanides is 5.6 Cal. Addition of silver chloride to its solution precipitates silver cyanide:

KAg(CN)2 + AgCl ⇔ 2AgCN + KCl.

A solution of the double cyanide is extensively employed as a bath for silver-plating.
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